The present invention relates generally to pulse width modulation (PWM) amplifiers. An amplifier circuit of this general type functions to convert a low frequency signal such as an audio signal into a pulse width modulated signal, to amplify the modulated signal and to demodulate the latter to obtain the original low frequency signal.
FIG. 1 shows an example of a pulse width modulation amplifier circuit in which an audio signal is applied to a non-inverting input terminal of an integrator 1 including a capacitor C.sub.1, and a carrier signal is applied from a rectangular wave oscillator 2 through a resistor R.sub.c to an inverting input terminal of the integrator 1. An output of the integrator is passed through a comparator 3 and a switching amplifier 4 to obtain a pulse width modulated signal which is passed through a low-pass filter 5 which demodulates the signal to thereby obtain an output audio signal. Further, the pulse width modulated signal is fed back through a resistor R.sub.N to the integrator 1 to form a negative feedback loop.
The circuit of FIG. 1 is referred to as a direct feedback system with which the overall frequency characteristic of the circuit is flat due to the passage of the input audio signal through the integrator 1. Moreover, the closed loop phase is within 90.degree. which provides good stability. However, since the closed loop includes the integrator 1, the amount of the negative feedback is 6 dB per octave. Hence, the higher the frequency, the higher the attenuation. Therefore, the signal distortion increases with an increase of the frequency causing the amplifier to be unsuitable to use for high fidelity purposes.
FIG. 2 is another example of a PWM amplifier circuit in which a carrier signal is obtained from a ramp wave oscillator 6 and a low-pass filter 7 is incorporated in a feedback circuit. The audio signal is derived from a pulse width modulated output signal from a switching amplifier 4. Negative feedback is provided to an amplifier 8 disposed on the input side of the circuit. The overall frequency characteristic of the circuit must be corrected by inserting a low-pass filter 9 having the same frequency characteristic as the filter 7 because the frequency characteristic increases from the roll-off point of the filter 7 due to the reverse nature of the bandpass charcteristic thereof. Further, in order to properly determine the attenuation of the carrier signal, it is necessary to set the cut-off frequency f.sub.e of the low-pass filter 7 sufficiently low in comparison with the carrier signal frequency. Therefore, it is difficult to obtain sufficient feedback in high frequency ranges. In addition to this, the frequency characteristic of the amplifier is not flat due to possible characteristic variations of the low-pass filters 7 and 9.
FIG. 4 shows an example of a push-pull output circuit of the switching amplifier 4 of the PWM amplifier of FIG. 1 or 2 in which complementary MOS transistors are used as switching elements. In this circuit, a drain of a P channel type transistor Q.sub.1 and a drain of an N channel type transistor Q.sub.2 are commonly connected to push-pull drive the load 11, which includes the low-pass filter 5 and a loudspeaker. Sources of the transistors Q.sub.1 and Q.sub.2 are supplied with power supply voltages of +V.sub.cc and -V.sub.cc, respectively, and gates thereof are supplied with positive and negative parts of the output signal from the comparator 3, respectively. In addition, an excess current detecting circuit is provided to detect excess current flow through the transistors Q.sub.1 and Q.sub.2 to thereby protect them. In this circuit, a resistor R.sub.1 is connected between the power source line and the source of the transistor Q.sub.1 to detect the current flowing through Q.sub.1. The voltage across the resistor R.sub.1 is sensed by a PNP transistor Q.sub.3 connected as shown. A detection output indicative of excess current is provided on the collector of the transistor Q.sub.3. A suitable protection circuit is operated with this detection output.
With the above-mentioned circuit construction, it is impossible to avoid power loss in the direction circuit because of the series connection of the resistor R.sub.1 to the current line resulting in a low overall power efficiency of the PWM amplifier.
An object of the present invention is thus to provide an excess current detecting circuit utilizing a power amplifying active element of switching type which is capable of detecting excess current without lowering the power efficiency of the power amplifier.